The invention relates to the field of gas generators, and more particularly to a gas, such as oxygen, concentrator for a breathing apparatus.
The use of on-board oxygen generating systems (OBOGS) technology for supplying breathing gas for aviators has been used for nearly 20 years. During this period, many pilots have complained about suffering from prolonged exposure to high concentration of oxygen in the breathing gas. The ASCC Advisory Publication 61/59 recommends that the inspired gas shall not exceed 60 percent oxygen with cabin altitudes below 15,000 ft. The present invention provides a means of controlling an OBOGS to produce the desired concentration of oxygen.
Oxygen generation for aircraft breathing applications requires that the product gas concentration stays within predetermined altitude-dependent minimum and maximum physiological limits. Normally, the minimum oxygen content of the breathable gas is that required to provide, at all cabin altitudes, the same or greater oxygen partial pressure as at sea level. A maximum oxygen concentration is set to reduce the likelihood of partial lung collapse during low-altitude high G maneuvers. More particularly, the risk of partial lung collapse increases with the risk of total adsorption of entrapped pockets of gas in the lungs, which result from distortion of the lungs during high G maneuvers. The risk of total adsorption of the entrapped gas increases with increased oxygen concentration (i.e. reduced nitrogen concentration).
Methods are known for the generation of low-pressure oxygen-enriched air. One such method is referred to as pressure swing adsorption (PSA) and has the advantage of being able to provide oxygen-enriched air in a short period of time after the supply of a suitable feed gas (e.g. pressurized air). The pressure swing adsorption process uses pressure to control adsorption and desorption. According to this process, the nitrogen in pressurized air is adsorbed in a molecular sieve bed while the oxygen passes through the bed.
When the molecular sieve in the bed has become nearly saturated with nitrogen, the bed is vented to atmospheric pressure. This causes most of the nitrogen-adsorbed gases to be desorbed and discharged from the bed. In a two-bed system, when one bed is producing oxygen, some of the enriched product gas is flushed back through the (vented) other molecular sieve bed to further lower the partial pressure of the adsorbed gases in the vented bed and to complete the desorption process. Using two beds that are pressurized and flushed alternately provides a continuous flow of product gas and ensures sufficient pressure for the flushing operation.
The known OBOGS are generally based on the molecular sieve gas separation process discussed above. Such systems are said to be xe2x80x9cself-regulatingxe2x80x9d since the pressure swing desorption increases with altitude, and therefore the efficiency of the process also increases to ensure sufficient oxygen concentration at high altitudes. More particularly, since each sieve bed is vented to the atmosphere (or cabin) during its regeneration phase, the bed pressure during desorption decreases with increasing altitude, thereby enhancing the desorption process.
In order to keep the oxygen concentration within maximum limits at low altitudes, processes have been developed to reduce the performance. There are five primary methods for reducing/altering PSA performance: Altering overall cycle time (disclosed in U.S. Pat. Nos. 4,661,124 and 5,004,485), altering relationship of fill to vent within a given cycle period (disclosed in U.S. Pat. Nos. 6,077,311 and 6,383,256 B1), artificially increasing product flow by bleeding product to ambient (disclosed in U.S. Pat. No. 4,567,909) or restricting/controlling fill opening, vent opening or both to manage pressure drop and flow.
Prior art OBOGS U.S. Pat. Nos. 4,661,124 and 5,004,485 (Hamlin, et al), disclose an alternating bed oxygen generating system with controlled sequential operation of charge and vent valves according to a series of selectable overall cycle times ranging between a minimum and a maximum, in a number of discrete steps. By extending the overall cycle time, efficiency of the system is reduced thereby regulating the product gas oxygen concentration to within physiological maximum limits.
In U.S. Pat. No. 4,661,124, the overall cycle time of the molecular sieve beds is controlled using a pressure transducer on the basis of cabin pressure that is indicative of the altitude at which the aircraft is operating.
In U.S. Pat. No. 5,004,485, an oxygen sensor is used to test the gas concentration and a comparator function is implemented to compare the sensed oxygen concentration with values in a look-up table of desired product gas oxygen concentrations at various altitudes. In response to implementing the comparator function the overall cycle time is controlled to provide suitable concentration levels.
Prior art systems employing overall cycle time control, such as disclosed in U.S. Pat. Nos. 4,661,124 and 5,004,485 (Hamlin et al) suffer from a disadvantage in that it is difficult to accurately control the output oxygen concentration because performance changes occur over a small range (e.g. 4.5 seconds to 5.5 seconds in some systems, whereas cycles ranging from 5.5 seconds to 8.5 seconds do not result in any performance changes).
Dynamic control of system performance to regulate product gas to within the minimum and maximum physiological limits requires reliable performance of the oxygen sensor connected to the concentrator output. U.S. Pat. No. 5,071,453 (Hradek, et al) discloses a Built-In-Test (BIT) function for implementing a system self-test for preflight and an oxygen sensor calibration check for operational level maintenance.
In U.S. Pat. Nos. 6,077,311 and 6,383,256 B1, the overall cycle time of each of the molecular sieve beds is maintained constant while that duration of the adsorption phase to the desorption regeneration phase is changed.
While the above-cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.
The present invention relies upon the ability of a slide valve or similar valve used in an OBOGS to remain in a third state which is neither completely open nor completely closed or control the rate/slew of the transition from state 1 to state 2. Furthermore, this third state should be able to be manipulated in order to provide a variable restriction to air flowing into the molecular sieve beds (charge) and depleted air flowing out of the molecular sieve beds (vent). Generally the restriction created by the less-than-completely-open valve results in a reduction of the separation efficiency of the molecular sieve bed and a reduction in the oxygen content of the product gas. The reduction in the product gas oxygen content is related to the amount of restriction created by the valve.
In accordance with the present invention, a molecular sieve gas concentration controller system controls and monitors a product gas generated by a known type of molecular sieve device that has at least n molecular sieve beds for separating a selected gas from an input gas supply. The molecular sieve control system includes m number of valves (where m is one or more) having at least 3 different states or positions. The valve further has an input for receiving the input gas supply and at least y(where y is two or more) outlets.
For example, a simple valve may have four ports: one inlet and three outlets. During operation the one inlet is for air supply, one outlet is for bed venting and two outlets are for flow of air into each of two beds: bed 1 and bed 2. The inlet is communicated to bed 1, bed 2 or both during operation. The vent is connected to the beds inversely to the inlet. When the inlet communicates with bed 1, the vent communicates with bed 2. When the inlet communicates with bed 2, the vent communicates with bed 1. The inlet and vent ports will not communicate due to the slide valve design. There are three states of the valve. State 1 is where bed 1 is fully open to the inlet (closed to the vent) and bed 2 is fully open to the vent (closed to the inlet). State 2 is where bed 2 is fully open to the inlet (closed to the vent) and bed 1 is fully open to the vent (closed to the inlet). State 3 is when the valve has moved partially from state 1 to state 2, causing variable amounts of restrictions in bed ports and vent port. The controller as a function of desired purity of product controls the amount of restriction.
A movement system, such as a piston or motor, controllably moves the valve from one state to another. A controller that is operably connected to the movement means varies the restriction in state 3 in a selected open passageway in the valve. The invention can be applied to sieve bed systems with valves independently controlling vent gas, input gas or outlet gas flows, but for simplicity of the patent description, dependent control of both inlet and vent gas flow is described.
As the valve is caused to cycle back and forth from state 1 to state 2, it can be stopped at state 3 for any amount of time up to nearly the full half-cycle of the valve""s motion in each direction. State 3 can occur in either transition from state 1 to 2, or from state 2 to 1. The diagram in FIG. 8 illustrates the state 3 of the slide valve motion (and where it is stopped) on the restriction of the flow-area of the valve. Note that the terms xe2x80x9cfully openxe2x80x9d and xe2x80x9cfully closedxe2x80x9d in this diagram refer to the inlet flow path for one bed. That is, when the slide valve is fully open to charge one bed, the vent path to that bed as well as the charge path to the other bed is fully closed. And, conversely, when the slide valve is fully closed to prevent charging one bed, the purge vent to that bed and the charge paths to the other bed are fully open.
Note that neither the frequency (or period) of the bed cycles nor the relative duration of the vent or fill to overall cycle time is changed using this method of varying product gas composition. This method provides a manner of generating a desired oxygen concentration by monitoring the oxygen concentration of the product gas and adjusting the amount of restriction created by the slide valve such that the resulting oxygen concentration is driven toward the desired value.